Sep 26th, 2011
Category: Articles
Reflectivity and Air Spaces
In order to retard heat flow by conduction, walls and roofs are built with internal air spaces. Conduction and convection through these air spaces combined represent only 20% to 35% of the heat which pass through them. In both winter and summer 65% to 80% of the heat that passes from a warm wall to a colder wall or through a ventilated attic does so by radiation.
The value of air spaces as thermal insulation must include the character of the enclosing surfaces. The surfaces greatly affect the amount of energy transferred by radiation, depending on the material’s absorptivity and emissivity, and are the only way of modifying the total heat transferred across a given space. The importance of radiation cannot be overlooked in problems involving ordinary room temperatures.
The following test results illustrate how heat transfer across a given air space may be modified. The distance between the hot and cold walls is 1-1/2″ and the temperatures of the hot and cold surfaces are 212 degrees and 32 degrees, respectively. In CASE 1, the enclosing walls are paper, wood, asbestos or other similar material. In CASE 2, the walls are lined with aluminum foil. In CASE 3, two sheets of aluminum foil are used to divide the enclosure into three 1/2″ spaces.
Conduction 21 BTUs
Convection 92 BTUs
Radiation 206 BTUs
TOTAL 319 BTUs
CASE 1, UNINSULATED WALL SPACE The surfaces of ordinary building materials, including ordinary bulk insulation have a low radiation or emissivity rate, and a heat ray absorption rate of over 90%. Air has low density, so conduction is slight (only 21 BTUs). Convection currents transfer 92 BTUs.
Conduction 21 BTUs
Convection 92 BTUs
Radiation 10 BTUs
TOTAL 123 BTUs
CASE 2, THE SAME WALL SPACE EXCEPT that the inner surfaces were lined with sheets of aluminum foil of 3% emissivity and absorptivity. Note the drastic drop in heat flow by radiation, from 206 BTUs to 10 BTUs. Conduction and convection are unchanged. The original total heat loss of 319 BTUs drops to 123 BTUs.
Conduction 23 BTUs
Convection 23 BTUs
Radiation 2 BTUs
TOTAL 48 BTUs
CASE 3, TWO SHEETS OF (5% EMISSIVE) ALUMINUM FOIL divide the wall space into 3 reflective compartments. Heat loss by radiation drops 94% from Case 1. The 2 interior sheets retard convection so that its flow falls 75%. Conduction rises only 2 BTUs; from 21 BTUs to 23 BTUs. The total heat loss drops 85% from Case 1.
Reflection and emissivity by surfaces can ONLY occur in SPACE. The ideal space is any dimension 3/4″ or more. Smaller spaces are also effective, but decreasingly so. Where there is no air space, we have conduction through solids. When a reflective surface of a material is attached to a ceiling, floor or wall, that particular surface ceases to have radiant insulation value at the points in contact.
Heat control with aluminum foil is made possible by taking advantage of its low thermal emissivity and the low thermal conductivity of air. It is possible with layered foil and air to practically eliminate heat transfer by radiation and convection: a fact employed regularly by the NASA space program. In the space vehicle Columbia, ceramic tiles are imbedded with aluminum bits which reflect heat before it can be absorbed. “Moon suits” are made of reflective foil surfaces surrounding trapped air for major temperature modification.
HEAT LOSS THROUGH AIR
There is no such thing as a “dead” air space as far as heat transfer is concerned, even in the case of a perfectly airtight compartment such as a thermos bottle. Convection currents are inevitable with differences in temperature between surfaces, if air or some other gas is present inside. Since air has some density, there will be some heat transfer by conduction if any surface of a so-called “dead” air space is heated. Finally, radiation, which accounts for 50% to 80% of all heat transfer, will pass through air (or a vacuum) with ease, just as radiation travels the many million miles that separate the earth from the sun.
Aluminum foil, with its reflective surface, can block the flow of radiation. Some foils have higher absorption and emissivity qualities than others. The variations run from 2% to 72%, a differential of over 2000%. Most aluminum insulation has only a 5% absorption and emissivity ratio. It is impervious to water vapor and convection currents, and reflects 95% of all radiant energy which strikes its air-bound surfaces.
HEAT LOSS THROUGH FLOORS
Heat is lost through floors primarily by radiation (up to 93%). When ALUMINUM insulation is installed in the ground floors and crawl spaces of cold buildings, it prevents the heat rays from penetrating down, reflecting the heat back into the building and warming the floor surfaces. Since aluminum is non-permeable, it is unaffected by ground vapors.
CONDENSATION
Water vapor is the gas phase of water. As a gas, it will expand or contract to fill any space it may be in. In a given space, with the air at a given temperature, there is a limited amount of vapor that can be suspended. Any excess will turn into water. The point just before condensation commences is called 100% saturation. The condensation point is called dew point.
VAPOR LAWS
1. The higher the temperature, the more vapor the air can hold; the lower the temperature, the less vapor.
2. The larger the space, the more vapor it can hold; the smaller the space, the less vapor it can hold.
3. The more vapor in a given space, the greater will be its density.
4. Vapor will flow from areas of greater vapor density to those of lower vapor density.
5. Permeability of insulation is a prerequisite for vapor transmission; the less permeable, the less vapor transfer.
The average water vapor saturation is about 65%. If a room were vapor-proofed, and the temperature were gradually lowered, the percentage of saturation would rise until it reached 100%, although the amount of vapor would remain the same. If the temperature were further lowered, the excess amount of the vapor for that temperature in that amount of space would fall out in the form of condensation. This principle is visibly demonstrated when we breathe in cold places. The warm air in our lungs and mouth can support the vapor, but the quantity is too much for the colder air, and so the excess vapor for that temperature condenses and the small particles of water become visible.
In conduction, heat flows to cold. The under surface of a roof, when cold in the winter, extracts heat out of the air with which it is in immediate contact. As a result, that air drops in temperature sufficiently to fall below the dew point (the temperature at which vapor condenses on a surface). The excess amount of vapor for that temperature that falls out as condensation or frost attaches itself to the underside of the roof.
Water vapor is able to penetrate plaster and wood readily. When the vapor comes in contact with materials within walls, having a temperature below the dew point of the vapor, moisture or frost is formed within the walls. This moisture tends to accumulate over long periods of time without being noticed, which in time can cause building damage.
To prevent condensation, a large space is needed between outer walls and any insulation which permits vapor to flow through. Reducing the space or the temperature converts vapor to moisture which is then retained. The use of separate vapor barriers or insulation that is also a vapor barrier are alternate methods to deal with this problem. Aluminum is impervious to water vapor and with the trapped air space is immune to vapor condensation.
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